1. Field of the Invention
This invention relates to a commercially advantageous process for the preparation of an alkyl carboxylate represented by the general formula R.sup.1 COOCH.sub.2 R.sup.2 by homologation reaction from an alkyl carboxylate represented by the general formula R.sup.1 COOR.sup.2, wherein R.sup.1 represents a hydrogen atom, an alkyl group or an aromatic group and R.sup.2 represents an alkyl group.
The homologation reaction of an alkyl carboxylate of this invention can be shown by the following general scheme: EQU R.sup.1 COOR.sup.2 +CO+2H.sub.2 .fwdarw.R.sup.1 COOCH.sub.2 R.sup.2 +H.sub.2 O
(wherein R.sup.1 is a hydrogen atom, a normal or branched chain alkyl group with 1 to 10 carbon atoms, or an aromatic group with 6 to 14 carbon atoms and R.sup.2 is a normal or branched chain alkyl group with 1 to 5 carbon atoms).
The alkyl carboxylates which can be prepared by this reaction may be exemplified by ethyl acetate which has a wide commercial use.
2. Description of the Prior Art
Ethyl acetate has been heretofore prepared by low-temperature condensation of acetaldehyde by the use of a catalyst such as an aluminum alcoholate. On the other hand, two processes for direct preparation of ethyl acetate have been recently proposed: one is the reaction of methyl acetate with carbon monoxide and hydrogen by using Fe, Co, Ru or Rh as catalyst (Japanese Patent Disclosure No. 59212/1979), and the other is the simultaneous reaction of dimethyl ether and methyl acetate or reaction of methyl acetate with carbon monoxide and hydrogen by using Ru as catalyst (J. Am. Chem. Soc., 100, 6238 (1978)). In view of the expected future shortage of petroleum resources and taking in consideration the fact that the preparation of methyl acetate starting material may not be necessarily dependent upon petroleum products, these processes may be regarded as superior to the process which use acetaldehyde as starting material which is usually derived from ethylene.
The former process utilizes a catalyst such as cobalt or iron in the presence of a halogen, a halide, a phosphine, or an amine as the promoter, but has the disadvantage that the reaction should be performed under drastic conditions because of the low activity of the catalyst. For example, when cobalt acetate is used as the catalyst in the presence of iodine, to obtain a practical reaction rate at a reaction temperature of within the range of 150.degree.-220.degree. C., it is necessary to use extremely high pressure, e.g. up to 1200 Kg/cm.sup.2. On the other hand, in the latter process which utilizes diiodotetracarbonylruthenium(II) complex (RuI.sub.2 (CO).sub.4) or tris(acetylacetonate)ruthenium(III) complex as the catalyst in the presence of an iodine compound, the reaction proceeds at a slower rate than the former process, and only an extremely low space-hourly yield is attainable. Moreover, when dimethyl ether and methyl acetate are used concurrently as the starting materials, the process tends to become complex in order to separate and recover the unreacted starting materials.
As described above, in the practice of the reaction of methyl acetate or dimethyl ether with carbon monoxide and hydrogen by the processes known in the art, the largest problem is the extremely low reaction rates of these reactions in comparison with usual carbonylation or homologation reactions of methanol and development of a catalyst which may be employed on a commercial basis has been earnestly desired.